Chicago police Superintendent Eddie Johnson said Thursday that officers will crack down on "large, unsanctioned street parties" of the sort that became the scene of several shootings this past weekend, one of the city’s bloodiest in more than two years.

While such street parties are nothing new in crime-plagued neighborhoods on the South and West sides, Johnson estimated 20 percent of shooting victims this past weekend were taking part in those gatherings. Tribune data show 74 people were shot, 12 fatally, across the city over the weekend while police counted 66 people shot within a slightly shorter time period.

On Tuesday Johnson announced that 430 officers have been added to patrols in some of the hardest-hit West and South side police districts, and that the number will jump to 600 by this weekend. Some of those officers will be asked to monitor unauthorized street gatherings, he said Thursday.

"We have a responsibility to ensure that any large gathering is safe and being conducted legally," Johnson said at a news conference at police headquarters, flanked by other top department brass. "This is to ensure the safety of the surrounding community."

The officers will be paying particular attention to large gatherings in five districts: Ogden, Harrison and Austin on the West Side and Calumet and Gresham on the South Side, police officials said. Johnson said the officers would be patrolling 30 "emergency hot spot dispersal zones," where such gatherings historically have occurred.

Police officials said the crackdown will last at least a month.

People at parties targeted by police will be told to disperse and face getting a ticket or being arrested if they refuse, officials said. To provide legal recourse for breaking up the gatherings, officers will be looking for people drinking alcohol outdoors, smoking pot, playing music too loud or otherwise engaging in illegal behavior, Johnson said.

"I would guess that a lot of those gatherings probably had a gang nexus to it and rival gangs saw them out there and decided to do what they did," Johnson said, referring to the sites of mass shootings last weekend. "And, unfortunately, in a lot of instances they don't care who they shoot. They just know that their rivals are over there and (they) shoot."

He said that these gatherings are nothing new, "but we had just a ton of them for some reason last weekend." And parties grow much quicker than they used to because of social media, he said.

"Years ago, a large gathering had to be advertised in order for it to become a large gathering," said Johnson. "Now, we'll have people livestreaming from a particular location, 'We're over here partying doing whatever we do,' and 10 people (would) turn into 200 just like that.”

Manpower in the targeted neighborhoods will be increased by having some beat officers work overtime, while tactical unit officers would have days off canceled. Officers from the department's fugitive apprehension unit, which works with deputy U.S. marshals to find wanted criminals, and various gang and saturation units would also have their days off canceled, Johnson has said.

According to Tribune data, last weekend marked the worst violence of any single weekend in Chicago since at least before 2016, a year in which homicides hit records unseen for two decades. And Sunday saw more victims shot in a single day since at least September 2011, when the Tribune began tracking every shooting in Chicago. For the entire day, 47 people were shot, including a stunning 40 during a seven-hour period early Sunday.

As of Tuesday, a police spokesman acknowledged that no one was in custody for any of the weekend shootings. An update on those investigations was not available.